Caution: I did that many years ago on our '67 Cal 34 which also had an Atomic-4. But only a few years later while motoring 35 miles uphill to Catalina island I lay down in the v-berth to take a nap. Dead flat calm, hatches and ports all open tiller-pilot on and friends in cockpit on watch. After about 15 minutes I knew something was wrong. Stumbled topside and friends said I looked terrible. Face very red and felt dizzy and nauseous with a raging headache.
Didn't know what caused it until we pulled the engine while we replaced the At-4 with a Universal 25 diesel (see: The Do-It-Yourself Diesel, SAIL Magazine, March, 1987). The dry portion of the exhaust, which I had fabbed out of 2" galvanized water pipe, was holed like Swiss cheese so badly I could fold it in half by just stepping on it. It had corroded through and exhaust leaked through the layer of asbestos (yes, asbestos) tape insulation. This was in the days before CO detectors were available.
Based on what happened, I wouldn't recommend using galvanized pipe on a dry exhaust. At the very least, PLEASE make sure you install and regularly test a carbon monoxide detector. I understand the ABYC now requires them on new vessels and if you ever have a survey, the surveyor will list that in Category A (mandatory).